But so what, is this really exciting news? Surely we’ve discovered loads of exoplanets (planets outside of our Solar System); there are hundreds of billions of stars in our galaxy and if they all have planets then there must be loads of exoplanets, right?

Well that’s what a lot of people, and scientists, assumed. But astronomers didn’t actually find the first exoplanet until 1992, and that was a planet orbiting a pulsar, so it was weird to say the least. The first exoplanet orbiting a star like our Sun was found in 1995. Since then, and particularly over the last ten years, the pace of discovery has increased, and as of today 518 confirmed exoplanets have been found.

The goal for many is to find planets like Earth, primarily made of rock (these are called terrestrial planets) which orbit at just the right distance from their stars to be at the right temperature to have surface liquid water, as this is judged to be the key to allowing life to become established and to flourish. Such an orbit is said to be in a ‘habitable zone’ or ‘goldilocks zone’; as it’s not too hot, not too cold.

But, the technology for finding exoplanets is still in its relative infancy, although its cutting-edge science, and astronomers currently face something called a ‘sampling bias’, which means that the techniques we use to find exoplanets are more likely to find specific types of planets, thus our current list of exoplanets is not a ‘representative sample’ i.e. not all types of planets are represented in the list of discoveries (see The Extrasolar Planets Encyclopaedia for details).

The sampling bias is for exoplanets which are big and orbit close to their stars, as astronomers look for evidence of exoplanets by looking at the way they affect the stars they orbit (I’ll explain more in a later post). Most found so far are called Hot Jupiters, they’re around the size of their namesake, or even much bigger, and typically orbit their stars between 2% and 50% of the distance Earth orbits the Sun, very close indeed! Such planets obviously can’t support Earth-like life, although they are interesting, mostly because astronomers didn’t expect to find gas giants so close to their stars.

Other types of exoplanets include Hot Neptunes (like Hot Jupiters, but surprisingly, more like Neptune) and hypothesised planet types include carbon planets, iron planets and waterworlds, which are primarily made out of carbon, iron and water respectively. In fact we may have a candidate for a waterworld in planet GJ 1214 b.

So what about terrestrial planets like Earth? Well so far a small number of super-Earths have been found. These are planets similar in mass to Earth (up to 10 times greater) but much lower in mass than gas giants like Jupiter. Most are thought to be rocky terrestrial planets, most are between 3 to 7 times more massive than Earth, but no confirmed super-Earths have been found so far orbiting in habitable zones. So none are yet definite candidates for Earth-like life.

But now we have Kepler. The Kepler space telescope was launched in 2009 with the mission to find more exoplanets, especially smaller Earth-like exoplanets. Kepler has found its first terrestrial planet, a scorched super-Earth, roughly 40% more massive than Earth and 60% more dense, thus its likely to be made primarily of iron and silicate rock, like home. The downside is that it orbits at 2% of the distance Earth obits our Sun, so it’s pretty hot (surface temperature around 1,800° C), hence it’s a scorched super-Earth. So no life. Or not as we know it Jim.

But its still an immensely exciting discovery, Kepler has shown it can find small exoplanets, and many more are likely to follow over the next few years, hopefully some of those will be Earth-like, orbiting Sun-like stars and will be found in the habitable zone. Such a discovery will prove that other planets like ours do exist, and could be possible habitats for life. This would be a huge step for astrobiology as a science. Fingers crossed for Kepler.